is more of a guideline, than an unbreakable law, as all rules, they can be broken. The rule of 3 colors is simple: pick one primary color. Then, pick two other complementary colors.
There are three components of color: hue – which is the color, saturation – the intensity of the color, brightness – the brightness of the color
As Mies van der Rohe, the uber famous german architect, used to say: less is more.